As is well known, a scanning confocal microscope (hereinafter referred to as scanning microscope) operates as follows: a specimen to be observed is irradiated with a laser beam via an objective lens; light reflected off the specimen under observation irradiated with the laser beam or fluorescence emitted from the specimen under observation excited by the laser beam is focused into a point-like shape via the objective lens again and an optical system; the focused light is detected by a detector and a point image is thus acquired. In this process, a two-dimensional image can be produced by using galvanometric mirrors, which are deflectors, to scan an entire two-dimensional XY plane in the specimen under observation.
FIG. 17 shows a specific path of the laser beam in a scanning microscope of this type. In FIG. 17, a laser beam incident along a direction A passes through a beam splitter 132 and is scanned two-dimensionally by two deflectors 133a and 133b. The laser beam then enters a pupil projection lens 134, passes therethrough, and is focused into a spot on the image plane of an objective lens (not shown). When the two deflectors 133a and 133b are caused to rotate (swing over an angle) to two-dimensionally scan the image plane of the objective lens, reflected light or fluorescence from a specimen that coincides with the image plane travels back along the same path described above, returns to the beam splitter 132, is reflected off the beam splitter 132, further passes through a detection optical system, is incident on a photodetector (not shown) formed, for example, of a photomultiplier, is converted by the photodetector into an electric signal, and allows the pixels of a monitor (not shown) to display a two-dimensional image in correspondence with the XY scan positions of the two deflectors 133a and 133b described above.
In a scanning microscope of this type, to produce a two-dimensional, unevenness-free image based on two-dimensional scanning of the laser beam, the center of the exit pupil of the objective lens always needs to coincide with the rotation center of the laser beam that passes through the objective lens irrespective of the rotation angle (swing angle) of the two deflectors. To this end, the rotation center of each of the deflectors only needs to be conjugate to the position of the exit pupil of the objective lens. Using the two deflectors to two-dimensionally scan the laser beam, however, prevents both the two deflectors from being always conjugate to the position of the exit pupil of the objective lens. To address the problem, in Patent Literature 1, a scanning microscope includes a first deflector that deflects a light flux along one direction, a second deflector that further deflects the light flux having been deflected off the first deflector in the same direction, and a driver that causes the first and second deflectors to rotate with the orientations of the deflectors in synchronization with each other, and the first and second deflectors are disposed so that the position of the center of deflection resulting from the motion of the light flux deflected by the second deflector is located in a position between the first and second deflectors and coincides with a position conjugate to the position of the exit pupil of the objective lens.
The exit pupil of an objective lens, which is theoretically an image-side focal position of the objective lens, is a concept also including a position determined in consideration of important items necessary for those skilled in the art to design the objective lens, such as aberrations and vignetting of the objective lens.